1. Field of the Invention
The invention relates to a method for fabricating an image sensor.
2. Description of the Prior Art
As the development of digital cameras and scanners progresses, the demand for image sensor also increases accordingly. In general, today's image sensors in common usage are divided into two main categories: charge coupled device (CCD) sensors and CMO image sensors (CIS). The application of CMOS image sensors has increased significantly for several reasons. Primarily, CMOS image sensors have certain advantages of offering low operating voltage, low power consumption, and the ability for random access. Additionally, CMOS image sensors are currently capable of integration with the semiconductor fabrication process.
The CMOS image sensor separates (i.e., classifies) incident light into a combination of light of different wavelengths. The light of different wavelengths is received by respective sensing elements and is subsequently transferred into digital signals of different intensities. For example, the CMOS image sensor can consider incident light as a combination of red, blue, and green light. Those wavelengths are subsequently received by photodiodes, and then transformed into digital signals. However, in order to separate incident light, a monochromatic color filter array (CFA) must be set above every optical sensor element.
Referring to FIGS. 1-2, FIGS. 1-2 are perspective views illustrating a method for fabricating an image sensor according to the prior art. As shown in FIG. 1, a semiconductor substrate 100 is provided, in which at least a photosensitive region 132 is defined on the semiconductor substrate 100. A plurality of photodiodes 122, CMOS transistors (not shown), and shallow trench isolations 120 surrounding the photodiodes 122 are formed on the semiconductor substrate 100. Each of the photodiodes 122 is electrically connected to at least one of the transistors, and the shallow trench isolations 120 are used as an insulator between two adjacent photodiodes 122 for preventing short circuit.
Next, a planarizing layer 102 is deposited on the photodiodes 122 and the transistors, and a plurality of dielectric layers 104,106 and patterned metal layers 124, 126 are formed on the planarizing layer 102. The metal layers 124 and 126 are formed on top of each shallow trench isolation 120 to avoid covering each photodiode 122, which further prevents scattering of incident lights and cross talk of signals while the lights are gathered at the photodiodes 122. The metal layers 124, 126 are also a part of the multilevel interconnects formed within the circuits of the CMOS transistor. Next, a passivation layer 108 is formed on the dielectric layer 106, and a silicon nitride layer 110 is deposited to prevent mist and other impurities from entering the device.
A plurality of color filters 128 typically composed of R/G/B filter patterns are formed on the silicon nitride layer 110, in which the color filters 128 are disposed specifically on top of each photodiode 122. A planarizing layer 112 is then deposited on the color filters 128, and a photosensitive material (not shown) composed of resin is coated over the surface of the planarizing layer 112. The photosensitive material is composed of I-line photoresist adapted for wavelength of 365 nm.
Next, an exposure process is conducted with a 365 nm UV light on the photosensitive material, and a developing process is performed thereafter to form the photosensitive material into a plurality of photosensitive blocks 130.
After the photosensitive blocks 130 are formed, as shown in FIG. 2, a reflow process is performed by exposing the image sensor 140 to high temperature for 5-10 minutes. The high temperature utilized during the process transforms the photosensitive blocks 130 to a plurality of microlenses 134, in which each of the microlenses 134 has a semi-circular surface. This completes the fabrication of a conventional CMOS image sensor.
It should be noted that in the conventional art, only one exposure process is conducted during the transformation stage of the photosensitive material into a plurality of photosensitive blocks. As the gaps between photosensitive blocks 130 are mostly dependent upon the resolution of the photolithography process, the current approach of utilizing only one exposure process for fabricating microlenses 134 still has the disadvantage of producing large gaps 138 between microlenses 134. This increase in gap size not only reduces the area for collecting light, but also lowers the color saturation of the image sensor 140. Hence, it has become an important task in the field to fabricate image sensors with minimal gaps between microlenses.